A duplex stainless steel has both an austenite phase and a ferrite phase in its structure of steel, and has been heretofore used as a high strength and high corrosion resistance material for a petrochemical plant material, a pump material, a chemical tank material, etc. Further, since duplex stainless steel is generally a low Ni component system, reflecting recent price inflation of metal source materials, it has drawn keen attention as a material having a lower alloy cost with less cost fluctuation than an austenite stainless steel which has been a mainstream stainless steel.
There are hot topics concerning a duplex stainless steel, namely development of a lean-type steel and an increase in demand of the same.
A lean-type steel is a steel grade, in which the content of expensive alloys is restrained compared to a conventional duplex stainless steel so as to reinforce further the advantages of low alloy cost. Examples thereof include those described in Patent Literature 1 and 2, which are standardized according to ASTM-A240, and the former corresponds to S32304 (representative components 23 Cr—4 Ni—0.17 N), and the latter to S32101 (representative components 22 Cr—1.5 Ni—5 Mn−0.22 N).
Conventional main steel grades, JIS-SUS329J3L, SUS329J4L, etc., are further corrosion-resistant than SUS316L, which is an austenite type highly corrosion-resistant steel, are added with expensive Ni and Mo in an amount of approx. 6 to 7% and approx. 3 to 4% respectively (hereinafter % with respect to a component means mass-%).
Compared with this, a lean duplex stainless steel compromises on the corrosion resistance level close to SUS316L or a standard type of steel of SUS304, but in exchange decreases greatly the contents of Mo to almost 0, and Ni to approx. 4% in the case of S32304 and approx. 1% in the case of S32101.
Further, a steel grade having decreased Ni and Mo, while maintaining corrosion resistance close to JIS-SUS329J3L, has been recently developed, and described in Patent Literature 3 and standardized as S82441 according to ASTM-A240. More precisely, referring to SUS329J3L cost reduction is attempted while securing the corrosion resistance by reducing Mo from approx. 3 to approx. 1.6, and Ni from approx. 6 to approx. 3.6, and in exchange increasing Cr from approx. 23 to approx. 24, Mn from approx. 1.5 to approx. 3, and N from approx. 0.15 to approx. 0.27.
Patent Literature 4 describes an improved grade of S32304 in Patent Literature 1, in which Cu is added to improve the corrosion resistance in an acidic environment and any of Nb, V and Ti is added in order to increase the strength. Further, Patent Literature 5 prescribes a lean duplex steel component system as an austenite-ferrite stainless steel superior in ductility and deep drawability, into which V is added at 0.5% or less as an optional element, which is allegedly an element for effectuating an increase in strength by means of micronizing the structure of steel.
A problem to be solved with respect to a lean duplex steel is decrease in corrosion resistance of a weld heat-affected zone. In a lean-type duplex stainless steel N is generally added in a large amount instead of Ni and Mo. In the case of such a high N duplex steel, if the material should have received in carrying out welding a heat input beyond a limit in a heat-affected zone (so-called HAZ) near the weld, extreme decrease in corrosion resistance may take place.
Consequently, a high N duplex steel is utilized in a limited way despite its low alloy cost in an application where corrosion resistance and toughness are less important, or as a structural material for welding with a low heat input, namely with a lowered welding speed.
To overcome the problem the inventors have disclosed in Patent Literature 6 a lean duplex stainless steel superior in corrosion resistance of a weld heat-affected zone and toughness, characterized in that the steel contains C: 0.06% or less, Si: 0.1 to 1.5%, Mn: 2.0 to 4.0%, P: 0.05% or less, S: 0.005% or less, Cr: 19.0 to 23.0%, Ni: 1.00 to 4.0%, Mo: 1.0% or less, Cu: 0.1 to 3.0%, V: 0.05 to 0.5%, Al: 0.003 to 0.050%, O: 0.007% or less, N: 0.10 to 0.25%, and Ti: 0.05% or less, wherein the balance is Fe and unavoidable impurities, and that the Md30 value is 80 or less, Ni-bal. is from −8 to −4, the upper limit of the N content is expressed by a relational expression with the Ni-bal., the austenite area ratio is from 40 to 70%, and 2×Ni+Cu is 3.5 or more. The key point of the invention is suppression of nitride precipitation in a HAZ by addition of a trace of V at a solid solution level, as well as by defining the upper limit of N in accordance with the Ni-bal. which is an austenite amount estimation formula.